330 O. E. MEINZER ESTIMATING GROUND-WATER SUPPLIES 



to outline in general terms the sources of the subterranean streams. This 

 encroachment on the realm of mystery seemed almost sacrilegious, and 

 has, naturally enough, been regarded by the ordinary man with' a good 

 deal of skepticism. However, water supplies are so necessary for nearly 

 all undertakings that no one can claim the vision of a prophet in this re- 

 spect without being compelled to prophesy. Thus, perforce, the science 

 of ground-water hydrology has been developed, and its development has 

 taken place under the wholesome stimulus of practical men, who insist 

 on specific predictions, and then immediately proceed to sink wells which 

 will definitely test these predictions. 



The progress in developing methods for estimating quantities of 

 ground-water has resulted largely from the demand for information re- 

 quired for the complete utilization of the ground-water resources in the 

 arid regions. In humid regions it is seldom desired to pump out of wells 

 all the available water that is in the earth, but in many irrigation dis- 

 tricts that is precisely the problem. Fortunately, the conditions are more 

 favorable for quantitative studies in arid than in humid regions. 



The study of water supplies differs from a study of other mineral de- 

 posits, in that it relates to dynamic, not static, conditions; it relates to 

 processes that are going on in the earth at the present time, not alone to 

 processes of past geologic ages. We have very little interest in mineral 

 reserves. The practical question with which we have to deal is not as to 

 the quantity of ground-water that lies below an area, but as to the rate 

 of replenishment, for conservative developments must be based on the 

 rate at which the ground-water can be withdrawn year after year, for 

 generations to come, without depleting the supply. 



It should be understood that this discussion relates only to the water 

 in the zone of saturation — ^that is, to the g^'ound-ivater, meaning the 

 basal, or bottom, water, also called phreatic water, which in Greek means 

 well water. It does not relate to the subsurface water that fails to get 

 down to the water-table, which would be a very different subject. 



Four Groups of Methods 



in general 



Four principal methods, or groups of methods, are used to determine 

 the annual recharge or "safe yield^' of ground-water. These may be 

 called the intake, discharge, water-table, and underflow methods. The 

 first of these consists in measuring the quantity of surface water that 

 seeps into the earth and percolates into the zone of saturation; the sec- 

 ond, in measuring the ground-water that is discharged through springs 



